Download Week 10

6/1/16
Week 10; Wednesday
Announcements:
Family ID final in lab Today
Final exam next Tuesday at 8:30 am here
Lecture: Speciation & Species concepts.
What are species?
Species are historically considered to be the basic unit of biodiversity, but what does
this mean?
Ever since Darwin, … Biologists have been debating the nature of species, even
debating the reality of species.
Speciation
The evolution of a new species from an existing species.
To say that volumes have been written on the topic of speciation would be a gross
understatement.
Let’s start with patterns, since that is what we can observe in nature.
Patterns of Speciation inferred from the fossil record:
A
B
C
B
C
time
time
A
B
A
phenotype
phenotype
Gradual
(Anagenesis)
Punctuated
(Cladogenesis)
From our discussion of phylogeny, it is apparent that in most cases speciation is
accompanied by phenotypic innovation (apomorphies), yet many discussions of
speciation dwell not on the evolution of phenotypic innovation, but on the evolution of
reproductive OR on the branching of lineages whether there is phenotypic change or
not.
This is part of the reason why the debate about speciation has been so
controversial over the years!
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Reproductive Isolation – Speciation requires that ‘gene flow,’ or exchange of genes
through sexual reproduction, between ancestor-descendant lineages of organisms
have ceased, or be extremely small, prior to the origin of a new species.
The term “reproductive isolation” often is used in two different ways leading to
confusion:
1) A history of no prior crossing or gene flow (little “r” reproductive isolation) – this
could be simply a consequence of geography or other physical isolation. This is
necessary for speciation to occur, but not always sufficient by itself
2) An evolved mechanism to prevent crossing (Big “R” Reproductive isolation) – this
is usually attributed to selection acting to create this mechanism to prevent gene
flow, but also could be a non-selective consequence of divergent evolution resulting
from #1. This is often considered sufficient for speciation to occur, but not always
necessary.
Does this mean that the fact of two lineages being non-interbreeding is sufficient to
say that they are species, even if no evident change has occurred?
“Sibling species” - result when reproductive isolation occurs without any
phenotypic innovation. These would only be considered species, IF you hold that
reproductive isolation is the defining basis for speciation.
Examples:
1) Behavioral isolation in Hawaiian Drosophila
2) autopolyploidy in plants (some plant species have 2n, 4n, 6n, 8n, 16n, etc.)
Phenotypic Innovation
When there is no fossil record, ‘speciation’ denotes the branch points in a phylogeny:
A
B
C
D
E
Speciation 'events'
Time
Does this mean that the fact of two lineages having diverged is sufficient to say that
they are species, even if no phenotypic change has occurred?
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Mechanisms of Speciation are usually conceived to be either:
Gradual
- accumulation of genetic differences
- selection for reproductive isolation through reduced fertility of hybrids, or
mechanisms that prevent crossing (for example, pollinator specificity)
OR
Sudden:
- polyploidy
- hybridization accompanied by chromosomal or ploidal change
- "quantum" or "macromutational" change – ‘hopeful monster’ of Goldschmidt,
perhaps including homeotic mutations – transform one organ into another
(e.g., loss of one pair of wings in insects)
How does speciation occur?
Two terms often used in discussions of speciation and reproductive isolation:
Allopatric speciation – reproductive isolation occurring in separate geographic
distributions
Sympatric speciation – reproductive isolation arises while occupying the same or
overlapping geographic distributions -- This is perceived to be rare and some say it
does not occur. It requires some mechanism to prevent reproduction between
individuals in close proximity.
Conventional (Selectionist) view:
A population in allopatry is subject to a different environment. Directional selection
drives the evolution of morphological novelty as an adaptation to the new
environment. At the same time the new “species” evolves differences that make it
less compatible with its progenitor and therefore is “reproductively isolated.”
Reinforcement, through evolution of physical or genetic barriers to crossing
Reproductive isolation, may occur if the two species come back into contact.
Alternative (Non-selectionist) view:
Release-from-selection. In this scenario, a population, most likely in
allopatry, is subjected to a relaxation in selection by virtue of being in a new
environment where some new resource is abundantly available, or some new niche
space is available. In this setting, phenotypic variability is greatly expanded and new
phenotypes at the extreme of the range of variability survive and reproduce that
normally would not. If one of these is turns out to be better adapted to the new
environment, then selection will rapidly fix the new phenotype.
This often is accompanied by a phenomenon called the “founder effect,” which
results in divergent phenotypes due to the chance loss of much genetic variability in
the small ‘founding’ population.
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- If the new environment has many available resources/niches, or if the evolutionary
novelty allows exploitation of previously unused resources, then many new forms
may arise in a short period of time - Phylogenetic Radiation
- Examples: - oceanic islands, such as Hawaii
- new adaptive zones, such as bats
A
A
B
B
Phenotype
Directional selection
mediated speciation
Release-from-selection
mediated speciation
- Key innovation is a concept that matches a novel trait with a sudden rapid
radiation of species (example: nectar spurs on flowers).
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Species concepts through time
“A species is what a competent taxonomist says it is” (quote attributed to Cronquist)
Typological species concept - Aristotle - Linneaus - pre-Darwin
- species were created in a perfect form
- variation = environmental imperfection
- "type" specimen and "type" species remain in taxonomy today.
Morphological species - post-Darwin
-species concept based entirely on morphology, including variation
- the range of variation is measured and species boundaries are defined by gaps or
morphological discontinuities.
- variation is genetic and environmental; there is no perfect ‘type’
- Numerical taxonomy is one manifestation of this concept.
- The so-called “evolutionary species concept” is an outgrowth of this concept
Process-dependent - species concepts - mid-20th century. These are derived from
notions of what processes are assumed to be responsible for speciation
- Biological species concept: reproductive isolation - this implies that selection
leads to reproductive isolation
- Mate-Recognition concept: mating specificity - this implies that selection favors
proper mate selection (isolation is a side-effect)
Phylogenetic species concepts - 1980’s-1990's
- species are identified by derived traits shared by organisms
- assumes that evidence of evolutionary novelty or phylogenetic divergence is
sufficient to define speciation having occurred
- evidence of a phylogenetic split (for example, DNA evidence only) is sufficient to
denote speciation
Continuing debate centers around these two pairs of questions:
1) Are species things that exist in nature (resulting from the process of evolution)?
OR
Are species human constructs (arbitrary units to be defined based on consistent
criteria by those who study them)?
2) Are species evolutionary lineages historically independent of other lineages? OR
Are species cohesive groups of organisms held together by some process?
These two philosophies derive from two very different ways of viewing the
relationship between nature and the role of taxonomy. What do you think
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